We operate on a BES with 3.7xx OS. I appear to be the only user that can keep the BB and the inbox synced. Most other users collect up to 500 e-mails on the BB over time when there is only about 60 in their inbox. Nothing works - wireless sync, hard sync, filing into PST - nothing. I have escalated this to our BES guys and they dunno what this is. Just an FYI

Don't forget that any email in Outlook that is MOVED to a PST or Folder that BIS doesn't track will stay on the BB for as long as the "Keep Messages" is set to on the BB, or if that's set to FOREVER, "orphaned" messages will "age off" as space is needed.
See another post on that here.